The present invention relates to a knocking detection system for an ignition timing control apparatus of an internal combustion engine, which has the functions of detecting a knocking signal generated in or outside of a cylinder due to the internal pressure of the cylinder of the internal combustion engine and regulating the ignition timing so as to suppress the level of knocking below a predetermined value.
Conventionally, an ignition timing control apparatus for internal combustion engines has been proposed in which, taking note of the ignition timing closely related to the generation of knocking, the knocking conditions of the engine are detected while at the same time advancing or delaying the ignition timing according to the knocking conditions, so that the engine is operated under a small knocking condition called "trace knock", thus improving the output or saving the fuel cost.
FIG. 1 shows an example of a conventional knocking detection system used with such an ignition timing control apparatus and detecting whether or not a knocking has occurred.
Specifically, reference numeral 2 designates a knocking sensor making up vibration detecting means, which may take any form as long as it is capable of detecting a high-frequency vibration component caused by a knocking. For example, this means may be of a type detecting a knocking from block vibrations, or detecting a sound or a pressure vibration of cooling water, and may employ a configuration disclosed in U.S. Pat. No. 3,822,583.
Numeral 4 designates a knocking detection system the output of which is used to advance or retard the ignition timing. The output of the knocking sensor 2 is amplified to a predetermined voltage level by an amplifier circuit 41.
Generally, only when a knocking is generated the high frequency component of the knocking appears at or near the maximum value of the cylinder internal pressure. Regardless of a resonance or non-resonance type, however, the output signal of the knocking sensor contains a vibration noise (such as valve seating noise or ignition noise) appearing at other than the maximum value of the cylinder internal pressure. With an increase of engine speed, the vibration noise increases, thus aggravating the S/N ratio. The effect of the noise is especially great under high engine speed and heavy load conditions. An example is shown in FIG. 2. FIG. 2 illustrates the engine speed of a straight-type 6-cylinder engine, which is changed under full load, and resultant vibration acceleration of the block in the absence of a knocking. This indicates that the noise component undergoes a great change.
In the conventional method of deciding whether or not a knocking has occurred, the output of the amplifier circuit 41 is integrated by an integrator circuit 42 to produce a background noise level, which is compared directly with the output of the amplifier circuit 41 by a comparator 43. Numeral 44 designates a decision circuit for producing a logical level "1" in response to a knocking and a logical level "0" in the absence of a knocking. It is naturally desirable that the noise level be not changed by the engine conditions. Thus the abovementioned variations of noise level becomes a cause of deteriorating the knocking detectability.